A Short Circuit Breakdown

The financial cost of downtime for organizations ranges drastically per industry, region, and size. Many different causes can create such downtime, ranging from natural causes like hurricanes and tornadoes, to human causes like protests or riots, or infrastructure causes such as an electrical short circuit.

Every organization should understand its downtime cost and its maximum tolerable downtime duration. When a short circuit happens, damage to critical electrical equipment may bring the organization to a standstill for days or even weeks. It’s important to understand how a short circuit occurs, its potential damage, and how to avoid short circuits by conducting or updating a short circuit electrical analysis to properly identify any deficiencies in the electrical system.

There are many occurrences on how an electrical short circuit develops. Some short circuits may go unnoticed for an extended period and have cleared, or isolated automatically before there are any issues. Others may be caused at a fraction of a second and completely paralyze the organization. Short circuits can occur anywhere along the electrical distribution system–at the main switchboard or at a remote motor.

Electricity consists of free-flowing electrons that flow along the outer area of conductors and are constrained within the insulating material, which is also called a dielectric. Insulating materials have a dielectric strength, which is a unit of measure in volts per meter. The dielectric strength of an insulating material is the maximum electric field it can withstand before breaking down. When the insulating material breaks down, its atomic structure is altered to a point where it is no longer insulating, but rather conducting. This electrical phenomenon is called dielectric breakdown.

Dielectric breakdown of electrical wires can be caused by overvoltages acting on the wire, such as transients or voltage surges. When a dielectric breakdown occurs, excessive heat is generated on the insulation to the point of physical deterioration, increasing the chance of a short circuit occurring.

A short circuit, by definition, is the accidental or intentional conductive connection through a relatively low resistance or impedance between two or more points of a circuit which are normally at different potentials. Important parameters to consider based on the definition above is low impedance and different potentials. In alternating current systems, impedance is the complete opposite of current. The second parameter, different potentials, refers to different voltages.

Once a short circuit is caused by a low impedance connection between two or more points at different voltages, the available electrical energy at that point will follow Ohm’s law where the current equals voltage over impedance (I = V/Z). This can reach short circuit ampacity values of 100,000 amperes or more and arc flash incident energy levels of greater than 40 calories per centimeters squared. The pressure from a 100,000-amperes arc can reach around 400 pounds per square foot at a distance of 3.3 feet, which is about ten times the value of wind resistance that walls are generally built to withstand.

An electrical engineering analysis is required to determine the available electrical energy. Specifically, the potential short circuit at every point on the system to properly select and size electrical equipment, devices, and conductors rated for the potential short circuit.

To determine the available short circuit, the electrical engineering analysis consists of three main factors. First, the available short circuit from the power utility company must be determined. Second, conduct a system impedance analysis at every electrical equipment and wire location. The final factor is to determine all additional contributing short circuit equipment, such as motors and generators, and what short circuit values they contribute.

Once the short circuit analysis is completed, and the data is presented, electrical equipment must be verified, selected, and sized to withstand the available short circuit. Determining the available short circuit is the keystone to providing a fully protected electrical system.

Metropolitan areas where there is high region-wide construction development, such as the DC metro area, will comprise of many existing facilities that are susceptible to large increases in the available short circuit from utilities. Existing buildings that were once designed to meet previous available short circuit may not meet up to date increases. Aging electrical equipment and conductors are prone to deterioration, such as corrosion and dielectric breakdown, which increase the chances of short circuits. By knowing the up to date values of the available short circuit, organizations will be better prepared to invest in their electrical infrastructure and avoid financial losses due to downtimes and outages.

About the Authors

Albert Malats, P.E., C.P.D.
Senior Electrical Engineer

As a Senior Electrical Engineer in Southland Engineering’s Mid-Atlantic office, Mr. Malats conducts short circuit, selective coordination, and arc flash analysis using power software. He believes in the involvement of other engineering disciplines for an integrated life safety, energy, and electrical understanding. He is an active member of IEEE, ASPE, and NSPE.